1. Field of the Invention
The present invention relates to a silicon nitride sintered body which is high in mechanical strength and fracture toughness and low in thermal conductivity and a method of manufacturing the same.
2. Related Art Statement
Since the silicon nitride sintered body is excellent in the mechanical strength, fracture toughness, thermal resistance, thermal shock resistance, etc., its uses as the thermal resistive high strength materials have been actively being developed.
Since silicon nitride is a substance having a strong covalent bonding property and has extremely poor sinterability itself, it is known that a sintering aid such as MgO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.2, ZrO.sub.2 or the like is generally added to obtain high density sintered bodies. Among them, as the sintered bodies being high in mechanical strength and fracture toughness, there are known from Japanese patent application Laid-open No. sho 59-149,981 silicon nitride sintered bodies containing oxides or oxynitrides of Sr, Mg, a rare earth element and Zr.
However, although the above-mentioned silicon nitride sintered bodies possess high mechanical strength and high fracture toughness, they have a drawback that the heat conductivity thereof is too high, for instance, when they are used as engine parts necessitating the heat insulation. That is, the thermal conductivity of the above-mentioned silicon nitride sintered bodies is unfavorably about 0.07 (cal/cm.sec..degree.C.), which is too high for the application to parts such as engine parts which need the heat insulating property.